The vast majority of the 6 million subscribers to World of Warcraft play on servers controlled by the creator of the game, Blizzard Entertainment. However, a few enterprising individuals have managed to get their hands on copies of the server program and are now running their own, independent servers.
These WoW gold servers are 'renegade' servers - if Blizzard ever found them, you can be sure they would be shut down! However, these servers do provide a service that other WOW servers do not - far more stringent user caps, a more encapsulated community, GM run events and in many cases a much lower fee to play (or perhaps no fee at all!)